NOx purification systems are known which use a selective catalytic reduction (SCR) method of using ammonia to reduce and remove nitrogen oxide (hereinafter referred to as NOx) in exhaust gas from a diesel engine (see, for example, Patent Document 1). Such an NOx purification system generally supplies ammonia by adding a urea aqueous solution to a denitration catalyst (hereinafter referred to as an “NOx catalyst”) via an addition valve to hydrolyze the urea aqueous solution.
If a urea aqueous solution addition system is abnormal, the following problems may occur. An excessive amount of urea aqueous solution may be added to increase the amount of ammonia slipping through the NOx catalyst or an excessively small amount of urea aqueous solution may be added to reduce an NOx purification rate. Examples of abnormality of the urea aqueous solution addition system include crystallization of the urea aqueous solution, blockage of the addition valve by dirt or the like, inappropriate opening and closing of the addition valve, a decrease in the supply pressure of a urea aqueous solution supply pump, and time degradation of the addition system.
Patent Literature 1 discloses the following technique. When a urea aqueous solution is added to an NOx catalyst to purify NOx, a target NOx purification rate is compared with the actual NOx purification rate. If the actual NOx purification rate is lower than the target NOx purification rate, correction is performed to increase the addition amount of urea aqueous solution. If the increase in the addition amount of urea aqueous solution fails to improve the actual NOx purification rate, the system is determined to be abnormal.